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The cessation comes as government forces have been making gains around Aleppo in the north

Russian jets are reported to have intensified attacks on Syrian rebel positions, hours before a cessation of hostilities is due to come into force.

Russia said it was continuing to bomb "terrorists" in parts of Syria.

Earlier, almost 100 rebel factions agreed to respect the truce, the main Syrian opposition group has said.

The High Negotiations Committee (HNC) said Free Syrian Army factions and the armed opposition had signed up to the truce from midnight (22:00 GMT).

The temporary "cessation of hostilities" involves government and rebel forces - but not the so-called Islamic State (IS) group and the al Qaeda-linked Nusra Front. On Friday, Nusra Front urged its supporters to intensify attacks against President Bashar al-Assad and his allies.

Meanwhile, UN envoy to Syria Staffan de Mistura said that peace talks would resume on 7 March, if the fighting stops and aid is delivered as planned.

Warring parties in Syria were meant to make their intentions known by midday Friday ahead of the pause in fighting.

Announcing the intentions of rebel factions, the HNC said the Syrian government and its allies must not use the "proposed text to continue the hostile operations against the opposition factions under the excuse of fighting terrorism".

Overnight, Russian air strikes which were "more intense than usual" hit rebel bastions including Eastern Ghouta east of Damascus, northern Homs province and western Aleppo province, the Syrian Observatory for Human Rights monitoring group said.

"It's more intense than usual," Observatory head Rami Abdel Rahman was quoted by news agency AFP as saying. "It's as if they [the Russians and the government] want to subdue rebels in these regions or score points before the ceasefire."

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The UN hopes the truce will allow more aid to reach people trapped in besieged area

The Observatory said the rebel-held Damascus suburb of Douma had also suffered heavy air strikes on Friday, killing eight people, four of them children.

It said the Syrian government had also shelled the area, which is a stronghold of the Army of Islam rebel group.

But Russian President Vladimir Putin said his forces were targeting IS, Nusra Front and other extremist groups designated as legitimate targets by the UN Security Council, adding that "the decisive fight against them" would "without doubt, be continued".

Turkey said the Russian and government strikes in the run-up to the cessation gave "serious concerns".

President Recep Tayyip Erdogan's spokesman, Ibrahim Kalin, said Turkey had "played an active role" in drafting the ceasefire but "the fact that Russian planes' bombardments and Assad's forces' attacks on the ground have been continuing... gives us serious concerns about the future of the ceasefire".

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Bombardments of Syrian cities continue to take their toll on the civilian population

The cessation has been brokered by the US and Russia but scepticism has lingered over the plan.

BBC Arab affairs editor Sebastian Usher says all sides have made it clear they will fight if attacked.